Problem 5.98

A point source of light with wavelength \(\lambda=0.50 \mu \mathrm{m}\) is located at a distance \(a=100 \mathrm{~cm}\) in front of a diaphragm with round aperture of radius \(r=1.0 \mathrm{~mm} .\) Find the distance \(b\) between the diaphragm and the observation point for which the number of Fresnel zones in the aperture equals \(k=3\).